Add TLS support for Trillian server (google#1525)

Signed-off-by: Firas Ghanmi <fghanmi@redhat.com>

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## HEAD
 
+* Add TLS support for Trillian: By using `--trillian_tls_ca_cert_file` flag, users can provide a CA certificate, that is used to establish a secure communication with Trillian log server. In https://github.com/google/certificate-transparency-go/pull/1525
+
 ## v1.2.1
 
 ### Fixes

trillian/migrillian/main.go

@@ -18,16 +18,20 @@ package main
 
 import (
 	"context"
+	"crypto/tls"
+	"crypto/x509"
 	"errors"
 	"flag"
 	"fmt"
 	"net/http"
 	"os"
+	"path/filepath"
 	"strings"
 	"time"
 
 	clientv3 "go.etcd.io/etcd/client/v3"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/insecure"
 	"k8s.io/klog/v2"
 
@@ -57,6 +61,7 @@ var (
 
 	maxIdleConnsPerHost = flag.Int("max_idle_conns_per_host", 10, "Max idle HTTP connections per host (0 = DefaultMaxIdleConnsPerHost)")
 	maxIdleConns        = flag.Int("max_idle_conns", 100, "Max number of idle HTTP connections across all hosts (0 = unlimited)")
+	tlsCACertFile       = flag.String("trillian_tls_ca_cert_file", "", "CA certificate file to use for secure connections with Trillian server")
 )
 
 func main() {
@@ -77,7 +82,11 @@ func main() {
 	}
 
 	klog.Infof("Dialling Trillian backend: %v", *backend)
-	conn, err := grpc.Dial(*backend, grpc.WithTransportCredentials(insecure.NewCredentials()), grpc.WithBlock())
+	creds, err := newTrillianTransportCredentialsFromFlags(*backend)
+	if err != nil {
+		klog.Exitf("Failed to get credentials: %v", err)
+	}
+	conn, err := grpc.Dial(*backend, grpc.WithTransportCredentials(creds), grpc.WithBlock())
 	if err != nil {
 		klog.Exitf("Could not dial Trillian server: %v: %v", *backend, err)
 	}
@@ -117,6 +126,32 @@ func main() {
 	core.RunMigration(cctx, ctrls)
 }
 
+// newTrillianTransportCredentialsFromFlags returns "creds" of type credentials.TransportCredentials to be
+// passed as credentials arguments to grpc.WithTransportCredentials. It configures TLS credentials
+// if a CA certificate file is specified, otherwise it uses insecure credentials.
+func newTrillianTransportCredentialsFromFlags(backend string) (credentials.TransportCredentials, error) {
+	var creds credentials.TransportCredentials
+
+	if len(*tlsCACertFile) > 0 {
+		tlsCaCert, err := os.ReadFile(filepath.Clean(*tlsCACertFile))
+		if err != nil {
+			return nil, err
+		}
+		certPool := x509.NewCertPool()
+		if !certPool.AppendCertsFromPEM(tlsCaCert) {
+			return nil, fmt.Errorf("failed to append CA certificate to pool")
+		}
+		creds = credentials.NewTLS(&tls.Config{
+			ServerName: backend,
+			RootCAs:    certPool,
+			MinVersion: tls.VersionTLS12,
+		})
+	} else {
+		creds = insecure.NewCredentials()
+	}
+	return creds, nil
+}
+
 // getController creates a single log migration Controller.
 func getController(
 	ctx context.Context,